This invention relates to a key switch for use in key input devices, for example, point of sale terminals and electronic cash registers.
Referring to FIG. 4, a typical conventional key switch 100 for a key input device includes a single piece housing 10 with a key stem guide sleeve 12. A key stem 14 fits slidably into key stem guide sleeve 12. A key cap support 24 joins a cup-shaped key cap 16 to key stem 14. Depressing key cap 16 causes key stem 14 to slide downward. A tip of a coil spring 18 set inside key stem 14 electrically connects stationary contacts 22, 22 of a flexible printed circuit ("FPC") 21 on a printed wiring board ("PWB") 20.
Conventional key switch 100 is susceptible to dust and water infiltration through a gap between an inner surface of key stem guide sleeve 12 and an outer surface of key stem 14. To eliminate such infiltration, a novel key switch 101, shown in FIG. 5 (see Japanese Utility Model Application No. 74274/'91)has been proposed. In key switch 101 a key cap support 26 connects key cap 16 and key stem 14. Key cap support 26 has a top annular groove 27 in its undersurface. A bottom annular groove 29 encircles the outside of a base of key stem guide sleeve 12. A boot 28, of elastic material such as synthetic rubber, fits between key cap support 26 and housing 10.
Boot 28 has a large-diameter cylindrical end portion 28b with a small outward flange 28a. Large-diameter cylindrical end portion 28b is contiguous with a tapering cylindrical middle portion 28c. Tapering cylindrical middle portion 28c is contiguous with a small-diameter cylindrical end portion 28d. Small outward flange 28a is inserted in top annular groove 27. An edge of small-diameter cylindrical end portion 28d is inserted in bottom annular groove 29. These insertions constitute a hermetic seal at the ends of boot 28 and thus prevent dust or water from infiltrating coil spring 18 and stationary contacts 22, 22, through a gap between the inner surface of key stem guide sleeve 12 and the outer surface of key stem 14. Boot 28 urges key cap 16 back to a home (upper) position and thus acts as a return spring.
Key cap support 26 consists of a disk 26a affixed to key stem 14 by means of, for example, a screw. Disk 26a has an integral short cylinder portion 26b at its periphery. Disk 26a also has an integral outwardly extending flange 26c and an integral inwardly extending flange 26d. Outwardly extending flange 26c is fitted into cup-shaped key cap 16 to support key cap 16 rigidly. Inwardly extending flange 26d engages small outward flange 28a of boot 28 to secure boot 28 against accidental detachment.
When stationary contacts 22, 22 of FIG. 5 are replaced with a membrane switch (not shown), then coil spring 18 serves to press, and thereby actuate, the membrane switch as key cap 16 is pressed downward. When key switch 101 is pressed, coil spring 18 is brought closer to the membrane switch until it touches it. As key switch 101 is further pressed, coil spring 18 is compressed. A relatively large distance must be traversed before a large enough restoring force builds in coil spring 18 to overcome the resistance of the membrane switch, thereby causing the membrane switch to close. This is a significant drawback of the prior art design as explained below with reference to FIG. 6.
FIG. 6 illustrates a force-displacement curve that is characteristic of key switch 101 of FIG. 5. The upper home position of key stem 14 is indicated by 0 on the horizontal axis. From point 0 through point S.sub.p to point S.sub.b, the restoring force depends entirely on the elastic deformation of boot 28. When the stroke distance exceeds point Sb, the force-displacement curve M.sub.a of coil spring 18 is superimposed on that of boot 28. Q.sub.a represents the point at which the membrane switch is actuated. F.sub.a represents the force required to actuate the membrane switch. S.sub.n represents the travel of key cap 16 from the point at which coil spring 18 makes contact with the membrane switch and the point of actuation. In other words, before the force corresponding to Q.sub.a is reached, a displacement equal to S.sub.n must be traversed. When the restoring force reaches the peak point P, boot 28 buckles, rapidly reducing the restoring force generated by boot 28. The restoring force falls up to point B, imparting a click-like feel to key cap 16. Further depressing key cap 16 compresses coil spring 18. The dashed line M.sub.a represents the restoring three versus displacement curve characteristic of coil spring 18 alone. The restoring force of coil spring 18 is applied to the membrane switch as coil spring 18 is compressed by displacement past point S.sub.b. The membrane switch is activated by a three of F.sub.a. The total restoring force from point B to point Q.sub.a is equal to the sum of the restoring forces of boot 28, line K.sub.a, and coil spring 18, line M.sub.a. The total restoring force reaches Q.sub.a at the displacement, S.sub.b +S.sub.n where the force F.sub.a is applied to the membrane switch.
The problem with the designs of key switches 100 and 101 of FIGS. 4 and 5 is that a shallow stroke of the key switch may fail to actuate the membrane switch. Thus, professional operators typing at high speed may tend to stroke such key switches without actuating the switch, causing errors in data input. This is a serious problem inherent in the prior art key switch structure.